Remove a redundant cast (#598)

protobuf/lib/src/protobuf/field_set.dart

diff --git a/protobuf/lib/src/protobuf/field_set.dart b/protobuf/lib/src/protobuf/field_set.dart
index f137aad..a7879cd 100644
--- a/protobuf/lib/src/protobuf/field_set.dart
+++ b/protobuf/lib/src/protobuf/field_set.dart
@@ -387,7 +387,7 @@
     assert(fi.index != null); // Map fields are not allowed to be extensions.
 
     var value = _getFieldOrNull(fi);
-    if (value != null) return (value as Map<K, V>) as PbMap<K, V>;
+    if (value != null) return value as PbMap<K, V>;
 
     var newValue = fi._createMapField(_message!);
     _setNonExtensionFieldUnchecked(meta, fi, newValue);